Tissue products such as paper towels, facial tissue, bath tissue, and other similar products have been formed in a variety of ways. A tissue product often has more than one layer to impart certain properties to the product. For example, the products may be formed from a multilayered paper web having an outer layer that gives the web a relatively soft feel. A variety of techniques have traditionally been used to form such multi-layered paper webs.
For example, some multi-layered webs have been formed by fully blending short fibers and long fibers to form an outer layer of the web. Moreover, other tissue products, such as described in U.S. Pat. No. 4,300,981 to Carstens, have been formed by utilizing primarily short fibers to form the outer layer so that the purity of the resulting short fiber outer layer can be substantially maintained. However, in some instances, minimal mixing has occurred within the layers such that 5% or less of the fibers within one layer comes from the fibers of an adjacent layer.
However, one problem associated with such conventional methods for forming multi-layered webs is that the webs do not have sufficient softness and/or strength. In particular, many of the conventional webs are soft, but lack sufficient strength in the -z direction. As a result, some fibers within the outer layer can break away from the web, thereby causing lint and slough.
As such, a need currently exists for an improved method of forming a paper web that is soft, but also possesses sufficient strength in the -z direction.
The present invention is generally directed to a method of forming a multi-layered paper web. In particular, the method of the present invention includes partially blending a first fibrous layer containing hardwood fibers with a second fibrous layer containing hardwood fibers and softwood fibers. Other fiber furnishes may also be utilized if desired.
In some embodiments, some of the fibers may be refined prior to forming the paper web. Refining can generally impart at least some strength to the web without substantially deteriorating bulk and/or stiffening the web. For example, one type of refining technique known as fibrillation can be utilized. When the fibers are refined, the extent of refinement can generally vary.
To form a paper web, in one embodiment, the fibers are supplied to a headbox that distributes the fibers to a papermaking machine. In one embodiment, to separate the fibers into layers, a headbox is provided that can include one or more dividers. For example, in one embodiment, a three-layered headbox is utilized that includes dividers that do not completely extend to the slice opening. In some instances, the dividers can be positioned so that the tips of the dividers are at least about 0.25 inches from the end of the slice opening, particularly from about 0.5 inches to about 10 inches, and more particularly between about 0.5 inches to about 7 inches.
In one embodiment, one layer within the headbox includes hardwood fibers and an adjacent layer within the headbox includes hardwood fibers and softwood fibers. Other headbox layers and/or fibrous materials may also be utilized. For example, in one embodiment, a three-layered headbox can include an outer layer containing hardwood fibers, an inner layer containing hardwood fibers and softwood fibers, and another outer layer containing softwood fibers and hardwood fibers.
In some embodiments, it may be desired that the fibrous layers be provided in approximately equal weights to aid in processing. For example, in some embodiments, a two-layered headbox has two fibrous layers that each are about 40% to about 60% of the weight of all the fibrous layers, particularly between about 45% to about 55%, and more particularly about 50%. Moreover, in other embodiments, a first fibrous layer containing hardwood fibers can be between about 30% to about 50% of the weight of all the fibrous layers, particularly between about 35% to about 45%, and more particularly about 40%. In addition, a second and third fibrous layer containing hardwood fibers and softwood fibers can each be about 20% to about 40% of the weight of all the fibrous layers, particularly between about 25% to about 35%, and more particularly about 30%.
In accordance with the present invention, the fibers from a first fibrous layer containing hardwood fibers can be partially blended with the fibers from a second fibrous layer containing softwood fibers and hardwood fibers. For example, in one embodiment, the fibrous layers are partially blended within a headbox. Once partially blended, the resulting web is formed with an outer layer having from about 5% to about 20% by weight of softwood fibers, particularly from about 5% to about 15%, and more particularly 10% to about 15% by weight of softwood fibers, which originate from the second fibrous layer.
By partially blending the fibers of one layer with the fibers of another layer, the resulting multi-layered paper web can have improved strength and yet remain soft. For example, partial blending can promote bonding of the fibers in the -z direction, thereby inhibiting the production of lint and slough, which typically results from weak -z directional bonds.
Other features and aspects of the present invention are discussed in greater detail below.